This application relates generally to combustors and, more particularly, to gas turbine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Most aircraft engines are able to meet current emission standards using combustor technologies and theories proven over the past 50 years of engine development. However, with the advent of greater environmental concern worldwide, there is no guarantee that future emissions standards will be within the capability of current combustor technologies.
In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures which do not allow the fuel-air reaction to proceed to completion (HC and CO). A small window exists where both pollutants are minimized. For this window to be effective, however, the reactants must be well mixed, so that burning occurs evenly across the mixture without hot spots, where NOx is produced, or cold spots, when CO and HC are produced. Hot spots are produced where the mixture of fuel and air is near a specific ratio when all fuel and air react (i.e. no unburned fuel or air is present in the products). This mixture is called stoichiometric. Cold spots can occur if either excess air is present (called lean combustion), or if excess fuel is present (called rich combustion).
Modern gas turbine combustors consist of between 10 and 30 mixers, which mix high velocity air with a fine fuel spray. These mixers usually consist of a single fuel injector located at a center of a swrirler for swirling the incoming air to enhance flame stabilization and mixing. Both the fuel injector and mixer are located on a combustor dome.
In general, the fuel to air ratio in the mixer is rich. Since the overall combustor fuel-air ratio of gas turbine combustors is lean, additional air is added through discrete dilution holes prior to exiting the combustor. Poor mixing and hot spots can occur both at the dome, where the injected fuel must vaporize and mix prior to burning, and in the vicinity of the dilution holes, where air is added to the rich dome mixter
Properly designed, rich dome combustors are very stable devices with wide flammability limits and can produce low HC and CO emissions, and acceptable NOx emissions. However, a fundamental limitation on rich dome combustors exists, since the rich dome mixture must pass through stoichiometric or maximum NOx producing regions prior to exiting the combustor. This is particularly important because as the operating pressure ratio (OPR) of moder gas turbines increases for improved cycle efficiencies and compactness, combustor inlet temperatures and pressures increase the rate of NOx production dramatically. As emission standards become more stringent and OPR""s increase, it appears unlikely that traditional rich dome combustors will be able to meet the challenge.
One state-of-the-art lean dome combustor is referred to as a dual annular combustor (DAC) because it includes two radially stacked mixers on each fuel nozzle which appear as two annular rings when viewed from the front of a combustor. The additional row of mixers allows tuning for operation at different conditions. At idle, the outer mixer is fueled, which is designed to operate efficiently at idle conditions. At high power operation, both mixers are fueled with the majority of fuel and air supplied to the inner annulus, which is designed to operate most efficiently and with few emissions at high power operation. While the mixers have been tuned for optimal operation with each dome, the boundary between the domes quenches the CO reaction over a large region, which makes the CO of these designs higher than similar rich dome single annular combustors (SACs). Such a combustor is a compromise between low power emissions and high power NOx.
Other known designs alleviate the problems discussed above with the use of a lean dome combustor. Instead of separating the pilot and main stages in separate domes and creating a significant CO quench zone at the interface, the mixer incorporates concentric, but distinct pilot and main air streams within the device. However, the simultaneous control of low power CO/HC and smoke emission is difficult with such designs because increasing the fuel/air mixing often results in high CO/HC emissions. The swirling main air naturally tends to entrain the pilot flame and quench it. To prevent the fuel spray from getting entrained into the main air, the pilot establishes a narrow angle spray. This results in a long jet flames characteristic of a low swirl number flow. Such pilot flames produce high smoke, carbon monoxide, and hydrocarbon emissions and have poor stability.
In an exemplary embodiment, a combustor for a gas turbine engine operates with high combustion efficiency and low carbon monoxide, nitrous oxide, and smoke emissions during low, intermediate, and high engine power operations. The combustor includes a center mixer assembly and a second mixer assembly radially outward from the center mixer assembly. The center mixer assembly includes a pilot fuel injector, at least one swirler, and an air splitter. The second mixer assembly is circumferentially outward from the center mixer assembly and includes a plurality of mixers that include a swirler, an atomizer, and a venturi. The combustor also includes a fuel delivery system including a pilot fuel circuit that supplies fuel to the center mixer assembly and a main fuel circuit that includes at least two fuel stages to supply fuel to the second mixer assembly.
During low power operation, the center mixer assembly aerodynamically isolates a pilot flame from a main stage of air. Under engine idle power operation, the combustor injects fuel only through the pilot fuel circuit directly into the center mixer assembly while channeling air through the second mixer assembly. Because the combustor operates using only the pilot fuel circuit during idle power operations, a high combustor idle power operating efficiency is maintained and combustor emissions are controlled. Under increased power operating conditions, fuel is injected through both the pilot and main fuel circuits. The fuel is dispersed evenly throughout the combustor to maintain control of emissions generated during increased power operations. As a result, a combustor is provided which operates with a high combustion efficiency while controlling and maintaining low carbon monoxide, nitrous oxide, and smoke emissions during engine low, intermediate, and high power operations.